For the second consecutive legislative session, disclosure of donors behind so-called dark money groups has surfaced as a key topic in the ethics reform debate for lawmakers frustrated with attacks stemming from secret donor cash.

Nonprofits allowed to spend unlimited sums on TV ads, mailers and other electioneering tools do not have to disclose their financial backers.

But a top House lawmaker slapped an amendment to require dark money disclosure on to what has been dubbed the most comprehensive ethics bill in decades.

The measure won final approval in the House on Wednesday, but Senate leaders have voiced vocal opposition to the idea of forcing politically active nonprofits to report donors.

The sweeping ethics bill, authored by state Sen. Van Taylor, R-Plano, now heads back to the Senate, where lawmakers approved a vastly different version last month and are all but certain to reject changes made by the House.

That leaves an array of big differences between the House and Senate proposals that need to be hashed out by a conference committee with little time left before the session ends Monday.

And there appears to be no compromise in sight on the political lighting rod that is dark money disclosure.

On Wednesday, state Rep. Byron Cook, a Republican from Corsicana who sponsored the ethics bill in the House, made clear the lower chamber doesn’t intend to budge on the issue.

“That is one of the signature pieces of any ethics legislation,” Cook said, noting that more than 120 House members supported a stand-alone piece of dark money disclosure legislation.

Asked if the House would be willing to bargain with the Senate on dark money, Cook responded sharply: “No.”

The Senate author was just as adamant in an interview Wednesday.

Taylor, who was tapped by Abbott to carry his signature ethics proposals for the session, said the upper chamber will not agree to send a bill to Abbott that includes a dark money provision.

“No chance,” he said. “Anonymous political speech absolutely must be something we protect.”

He added: “I’m going to work on the common ground that we’ve established and try to put together something that will pass that both houses … and send it to the governor’s desk.”

The ethics bill would expand personal financial reporting requirements, seek to tighten conflicts of interest involving lawmakers and require the disclosure of contracts worth more than $10,000 per year between elected officials and governmental entities.

But dark money has become the most controversial component. The House bill would require groups making independent expenditures of $25,000 or more to disclose the names of donors who give $2,000 or more.

State records show a small mix of conservative and liberal entities would fall into that category based on their expenditures during 2014.

A similar proposal was vetoed last session by Gov. Rick Perry, who said it would chill free speech.

Cook said he’s looking for Abbott to take a “position recognizing if we don’t address dark money one day this state will have a scandal like other states have had on this issue and we will be addressing it reactively as opposed to proactively.”

However, that is not likely, considering that Abbott, who declared ethics reform an emergency item in January, has said he believes donor and membership lists are constitutionally protected under a 1958 U.S. Supreme Court ruling involving the NAACP.

His stance on the House bill is still unknown. Abbott’s office released a statement when the bill measure cleared the Senate last month, applauding the ethics reform. But he’s remained mum publicly on the House bill.

Commenting on the potential for gridlock between the two chambers on the ethics bill, House Speaker Joe Straus said Wednesday that the lower chamber “will work to find common ground with the Senate on meaningful ethics reform legislation.